Rubidium-strontium age studies and geochemistry of acid veins in the Freetown complex, Sierra Leone
Rubidium-strontium age studies and geochemistry of acid veins in the Freetown complex, Sierra Leone
复制标题
塞拉利昂弗里敦复合体酸性矿脉的铷-锶年龄研究和地球化学
DOI：
10.1180/minmag.1977.041.320.12
复制
发表时间：
1977
影响因子：
2.7
通讯作者：
M. K. Wells
中科院分区：
文献类型：
--
作者：
R. D. Beckinsale;J. F. W. Bowles;R. Pankhurst;M. K. Wells
摘要塞拉利昂弗里敦侵入体的地层年龄界限很广，然而，尽管进行了多次尝试，但该侵入体以前并未通过同位素方法准确测年。与侵入体长时间冷却历史的早期阶段同生的酸性脉的Rb-锶测年结果为193±3 Ma。矿脉由石英和斜长石组成，并带有来自寄主辉长岩的残留矿物，主要是斜长石。对矿脉蚀变矿物的电子探针分析，以及矿脉和邻近寄主辉长岩的岩相学分析清楚地表明，矿脉是由花岗岩组分形成的，在初始脉内的含水流体相的帮助下，与周围的固体辉长岩就地分异。这一断言得到了从酸性岩石和基性岩石中获得的相同的、较低的初始87Sr/86Sr比值(0·70389)的支持，这里描述的技术可能对其他类似的侵入体测年有用。
Summary The stratigraphical limits on the age of the Freetown intrusion, Sierra Leone, are very wide, yet the intrusion has not previously been accurately dated by isotopic methods, despite a number of attempts. Rubidium-strontium dating of acid veins contemporaneous with the early stages of the prolonged cooling history of the intrusion provides an age of 193 ± 3 Ma. The veins consist of quartz and orthoclase with relict minerals, principally plagioclase, from thehost gabbro. Electron-microprobe analysis of the altered minerals of the veins, and the petrography of the vein and adjacent host gabbro clearly demonstrate that the veins were formed from a granitic fraction, differentiated in situ from the surrounding solid gabbro with the assistance of a hydrous fluid phase within the incipient vein. This assertion is supported by the identical, low value of the initial 87Sr/86Sr ratio (0·70389) obtained from both the acid and basic rocks, and the technique described here may be useful in dating other, similar, intrusions.